1. Field of the Invention
The present invention relates to interferometers for making highly accurate measurements of wavefront aberrations, particularly to phase-shifting point diffraction interferometers and methods for calibrating them.
2. State of the Art
Optical metrology is the characterization of surfaces and materials using optical methods. An area of optical metrology relevant to the present invention is the use of an interferometer to measure the quality of a test optic, such as a single or multiple element mirror or lens system.
One important recent application of optical metrology is the testing of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. In pursuit of this goal, systems are confronted with the diffraction limit determined in part by the wavelength of the light employed. To meet the challenge of imaging ever smaller features, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and may eventually move to even shorter wavelengths such as extreme ultraviolet or to yet shorter X-ray radiation.
For the extreme case of X-ray lithography, a proximity method that does not require re-imaging optics is under development. In X-ray proximity lithography, feature sizes are considerably larger than the wavelength limit. However, reticles in this case are required to have feature sizes equal to the desired printed feature size, currently on the order of 0.1 microns and smaller. It is quite difficult and expensive to manufacture reticles having such small feature sizes. Additionally, radiation passing through the reticle's narrow slits and apertures still diverges despite the extremely short wavelength. Thus, the reticles must be placed very close to the wafer, sometimes as close as a few microns, so that the shadow-cast image of the reticle remains sharp on the wafer. These systems must be carefully designed such that the reticle never contacts the wafer, an event that could destroy the reticle.
Because of the difficulties posed by proximity imaging a reticle pattern onto a wafer, it is desirable to extend the concepts of projection optics as currently used in visible-light or deep-ultraviolet lithography systems to even shorter wavelengths such as extreme ultraviolet. Such systems employ lenses or other optical elements to project a demagnified image of the reticle onto the wafer surface. This allows reticles to retain larger feature sizes, thus reducing the expense of generating the reticle itself As with all optical imaging systems, various aberrations such as spherical aberration, astigmatism, and coma may be present. These aberrations must be identified and removed during the fabrication and/or alignment of the projection optics, or the projection optics would introduce substantial blurring in the image projected onto the wafer.
In order to test the projection optics for various aberrations, interferometers may be employed. Conventional interferometers, based upon the Michelson design for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram, and the resultant wavefront with, for example, Zernike polynomials, indicates the presence of aberrations.
The reference wave of the interferometer should be "perfect"; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optical elements through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical elements through which the reference wave passes. While these artifacts, in theory, can be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly clean interferogram.
To address this problem, "point diffraction interferometers" have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer (PS/PDI) described in H. Medecki, et al., "Phase-Shifting Point Diffraction Interferometer", Optics Letters, 21(19), 1526-28 (1996), E. Tejnil, et al., "At-Wavelength Interferometry for EUV Lithography," J. Vacuum Science & Tech. B, 15, 2455-2461 (1997), K. A. Goldberg, et al., "Characterization of an EUV Schwarzchild Objective Using Phase-Shifting Point Diffraction Interferometry," Proceeding SPIE, 3048, 264-270 (1997), E. Tejnil, et al., "Phase-Shifting Point Diffraction Interferometry for At-Wavelength Testing of Lithographic Optics," OSA Trends in Optics and Photonics: Extreme Ultraviolet Lithography, Optical Society of America, Washington, D.C., 4, 118-123 (1996), K. A. Goldberg, "Extreme Ultraviolet Interferometry," doctoral dissertation, Dept. of Physics, Univ. of California, Berkeley (1997), and in the U.S. Patent Application "Phase-Shifting Point Diffraction Interferometer," Inventor Hector Medecki, Ser. No. 08/808,081, filed Feb. 28, 1997, which are all incorporated herein by reference.
The PS/PDI is a variation of the conventional point diffraction interferometer in which a transmission grating has been added to greatly improve the optical throughput of the system and add phase-shifting capability. In the PS/PDI, as illustrated in FIG. 8A, the optical system 2 under test is illuminated by a spherical wave 5 that is generated by an entrance pinhole 6 in a mask 4 that is placed in the object plane of the optical system 2. To assure the quality of the spherical wave illumination, pinhole 6 is chosen to be several times smaller than the resolution limit of the optical system. Grating 8 splits the illuminating beam 5 to create the required test and reference beams 10 and 12, respectively. A PS/PDI mask 20 is placed in the image plane of the optical system 2 to block the unwanted diffracted orders generated by the grating 8 and to spatially filter the reference beam 12 using a reference pinhole 16. The test beam 10, which contains the aberrations imparted by the optical system, is largely undisturbed by the image plane mask by virtue of it passing through a large (relative to the point spread function of the optical system) window 14 in the PS/PDI mask 20. The test and reference beams propagate to the mixing plane where they overlap to create an interference pattern recorded on a CCD detector 18. The recorded interferogram yields information on the deviation of the test beam from the reference beam which in the ideal case is a spherical wave. FIG. 8B depicts a PS/PDI mask 21 comprising a square shaped window 22 and a reference pinhole 24 to the right which has a diameter of less than 100 nm.
If the optical system under test were perfect (i.e., generating a perfect spherical wave) the interferogram would consist of a hyperbolic fringe field due to the fact that the interference pattern arises from the addition of two laterally-sheared spherical waves. These hyperbolic fringes lead to apparent aberrations in the resultant wavefront. Furthermore, when the optical system is not perfect, the quality of the reference beam depends upon how well the reference pinhole spatially filters the aberrations imparted by the optical system under test. These two effects limit how accurately an uncalibrated PS/PDI can measure the wavefront of an optical system under test.
As is apparent, the accuracy of the PS/PDI measurement system comes from the pinhole-diffraction-generated reference and illumination beams. This type of high-accuracy interferometer can be implemented in any spectral regime. The theoretical accuracy of the PS/PDI can be inferred by way of rigorous diffraction theory but, in practice, it is preferable to determine the accuracy and calibrate an interferometer based on actual measurements.
In order to experimentally calibrate and characterize the accuracy of the PS/PDI, a null test is performed. The null test consists of replacing the standard PS/PDI mask with a so-called "null-mask". In the null-mask, the large test beam window of the PS/PDI mask is replaced by a second pinhole, identical to the reference pinhole. Instead of interfering the aberrated wavefront from the optic under test with a reference spherical wave (spatially filtered version of the test beam), two spherical waves (spatially filtered versions of the test beam) are interfered. Under ideal conditions, the two wavefronts will be truly spherical and the primary aberration seen in the resultant wavefront will be coma due to the hyperbolic fringe field created by two interfering spherical waves. Other aberrations present in the measurement are indicative of systematic and random errors in the system. The aberrations remaining, after removal of the systematic effects attributable to the interferometer geometry, limit of the accuracy of the interferometer.
The major difficulty in performing the null test on the PS/PDI is the required alignment of two focused beams onto two pinholes that are smaller than the diffraction-limited resolution of the optic under test. Experimental results have shown the null test results to be susceptible to misalignment. The required alignment tolerance was found to be on the order of one-tenth of the diffraction-limited image point size, which for typical EUV lithographic optics corresponds to an alignment tolerance of 15 nm or smaller. In the conventional PS/PDI configuration used for optical system testing, as illustrated in FIGS. 8A and 8B, the alignment procedure consists primarily of aligning the reference beam to the reference pinhole. Because the test beam window is large relative to the image point size and is on par with the image point separation, the exact separation and rotational orientation of the two image points is not critical. For the null test alignment on the other hand, the two image points have to be aligned to two pinholes simultaneously. In this case, image point separation and orientation become critical, thus the alignment procedure in the null test mode is significantly more challenging than is the alignment procedure in the conventional optic system testing mode. The art is in search of a fast, repeatable, and systematic method for performing the alignment required to implement the null test.